It is common practice in many industries, such as the hotel and catering industries, for chemicals such as those used for cleaning to be purchased as concentrated liquids and then diluted with water to give the correct concentrations for use. Proportioning dispensing apparatus have been designed to achieve the desired dilution of the concentrated solution and dispense the mixed diluted solution.
These dispensers have commonly employed so-called venturi-type devices, known as eductors, to aspirate or draw the concentrated solution into the water stream. In these eductors water traveling at a high velocity through a passage entrains and dilutes the concentrated solution at a point where a restricted flow channel in the passage widens. These dispensers are generally operated with water provided directly from the main water supply. As it is important to maintain the water supply free of contamination, the eductors normally employ an air gap to prevent backflow of the chemicals into the water source.
Typically, air gap eductors operate in an upright arrangement, i.e., a water jet is directed vertically downwards, across an air gap, thereafter passing into and through a venturi structure where the concentrated liquid is entrained. Thus the eductor generally comprises a nozzle to generate the jet of water which passes downward through the air gap and to the venturi below the air gap. The venturi has an inlet located on an upper surface to receive the jet of water. Not all the water in the jet passes into the venturi inlet because the inlet usually has a smaller cross section than the jet. For example, a water jet with a diameter of 2-4 mm may be used with an inlet having a diameter of 1.5-1.8 mm. This helps to ensure that sufficient pressure is generated in the venturi. There is therefore a fraction of the water which does not pass directly into and through the venturi structure. This excess water must be collected from the top of the venturi structure and directed to the exit of the venturi. Eductors have been provided with bypass channels to enable excess water to pass around the venturi structure and join the water emitted from the exit of the venturi structure. For example a bypass channel may be formed by providing an outer discharge tube around the venturi structure and spacing the outer discharge tube from the outer walls of the venturi to provide a throughflow space.
It is possible that under some conditions water in the discharge tube travels backward, i.e., up through the discharge tube and out the entrance into the discharge tube. If this happens, a jet of water containing the diluted chemical can travel upward, cross the air gap, and contact the water jet nozzle at the top of the air gap. Since the water jet nozzle is connected to the source of water supply, it is possible that the chemical can migrate into the water supply.